The present invention relates to a method of producing an air-laid absorbent structure that includes hydrophilic fibres and particles of superabsorbent material. Such absorbent structures are used in sanitary articles, such as diapers, incontinence guards, sanitary napkins and like articles.
An absorbent structure in a sanitary article of the aforesaid kind intended for one-time use only is typically comprised of one or more layers of hydrophilic fibres, normally cellulose fluff pulp. The structure will also often include so-called superabsorbents, which are polymers that can absorb many times their own weight in water or body liquid. Such an absorbent structure is flexible and comfortable to wear, and also has a high absorption capacity.
One drawback of these known absorbent structures is their relatively low strength properties, particularly tensile strength. which can sometimes cause problems in the various steps of manufacturing the sanitary article in question and also during the use of said article. Various attempts have been made to enhance the strength and the structural coherency of such absorbent structures. among other things by mixing-in thermoplastic synthetic fibres with subsequent heating of the absorbent structure, c.f. for instance U.S. Pat. No. 4,590,114. The thermoplastic fibres are therewith melted and contribute towards a more coherent structure that has improved strength properties. The drawback with this solution is the relatively high price of the thermoplastic fibres and the negative influence that this so-called thermobonding has on the absorption properties.
A method of producing a wet-laid absorbent structure of relatively high tensile strength is disclosed in U.S. Pat. No. 4,551,142. for instance. This method involves providing an aqueous dispersion of cellulose fibres and superabsorbent particles, formation of this dispersion into a wet-laid sheet which is then dewatered, dried and compressed to the desired density. The procedure and equipment required in this process are completely different to those normally applied in the air-laying method, in which cellulose pulp in bale or sheet form is dry-shredded to form so-called fluff pulp and air-laid to form a pulp mat together with superabsorbent particles.
U.S. Pat. No. 5,516,569 discloses that an absorbent composite containing 40 to 85% superabsorbent particles is wetted to a moisture content of between 15 to 30% in order to bind the superabsorbent particles in the composite.
The object of the present invention is to provide a method of producing an absorbent structure formed in accordance with an air-laying method and having considerably improved strength properties. This object is achieved in accordance with the invention by moistening the air-laid structure to a moisture content of at least 15%, preferably at least 20%. and more preferably at least 35%, calculated on the total weight of the structure, and thereafter drying the structure to a moisture content of at most 12%, preferably at most 10%.
When the absorbent structure is moistened or wetted to a high moisture content, water may be pressed from the structure prior to drying the same.
The structure is preferably moistened with distilled water or de-ionized water.
The inventive method is flexible, by virtue of the fact that it is only necessary to moisten or wet certain parts of the structure in order to obtain better strength. For instance, the structure can be moistened in a suitable moistening pattern, e.g. in a strip pattern or network pattern. In the case of profile structures where different parts of the structure have mutually different weights per unit area, it may be suitable to moisten only those parts that have the lowest weight per unit area and therewith the lowest strength.